Updated iRel.h with iterator support

[Tim Tautges]

Hi all,
   Attached is an updated header for iRel with iterator support.  There's one very important assumption I'm making which 
I'm not sure is valid, and which I need input on: can one use an iterator to recover the set it's iterating?  Currently 
I don't see such a function.  The result is that very few iterator-based functions actually make sense for iRel, namely 
2 of them: getEntSetIterRelation and getEntArrSetIterArrRelation.

One other side effect of this change, and one which impacts both iGeom and iMesh, is that we need an 
iBase_EntityIterator type, which before now we didn't have.  That shouldn't be a terribly difficult change for most 
implementations.

And, just writing the signatures for these functions reminded me just how narrowly targeted these modifications are; for 
example, nowhere in our interfaces do we consider the notion of iterating over sets stored in a set.  Thus, 
iterator-based functions in iRel will not be useful here.  It's also not clear what the semantics are for creating an 
iterator over a set that contains sets.  Does that iterator iterate the entities contained in any sets?
You might ask who would ever want to do such a thing, but that's exactly the question we should avoid asking when 
designing interfaces.

- tim

-- 
================================================================
"You will keep in perfect peace him whose mind is
   steadfast, because he trusts in you."               Isaiah 26:3

              Tim Tautges            Argonne National Laboratory
          (tautges at mcs.anl.gov)      (telecommuting from UW-Madison)
          phone: (608) 263-8485      1500 Engineering Dr.
            fax: (608) 263-4499      Madison, WI 53706

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